Serum radioimmunoassay indicated the presence of carcinoembryonic antigen (CEA) in seven of 61 patients with inflammatory bowel disease. The only patient with a persistently positive assay result had proven colonic adenocarcinoma complicating chronic ulcerative pancolitis. Six patients without evidence of colonic cancer had transient antigenemia, which occurred in four during acute onset or flare-up of disease and disappeared with remission in three of the latter. Thus, a transiently positive CEA level in a patient with inflammatory bowel disease does not necessarily indicate the presence of colonic cancer.